better errors and error locations

frog/type.dart

 // Copyright (c) 2011, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 class Type implements Named, Hashable {
   final String name;
   bool isTested;
 
   /**
    * For core types (int, String, etc) this is the generated type assertion
@@ skipping 1125 matching lines @@
     if (ret != null) return ret;
 
     Member member = getMember(memberName);
     if (member == null) {
       // TODO(jimhug): Check for members on subtypes given dart's dynamism.
       return null;
     }
 
     // TODO(jimhug): Move this adding subtypes logic to MemberSet?
     ret = new MemberSet(member);